What's the difference between rem and em in CSS?

Understanding the Difference Between rem and em in CSS

In web development, it is crucial to manipulate text sizes to achieve the desired visual appeal. CSS provides various units for this purpose, and two commonly used options are em and rem. While both units play a similar role, they operate based on different principles, leading to distinct effects.

Em: Relative to Parent's Font Size

The em unit is primarily used to adjust the size of child elements relative to their parent element's font size. For instance, in the HTML and CSS code provided, both the div and the p element are set to have a font size of 1.4rem. If the default font size for the document is 16px, the div element will have a font size of 1.4 times 16px, which is 22.4px.

Rem: Relative to Base Font Size

Unlike em, the rem unit is not dependent on the parent element's font size. Instead, it refers to a specific base font size, typically defined in the root element (html). By default, browsers consider 1rem to be equivalent to 16px. In the example provided, the div and p elements both have a font size of 1.4rem. This means that they will have a font size of 1.4 times 16px, regardless of any changes to the font size in intermediate containers.

Key Difference: Inheritance vs. Absolute Referencing

The fundamental difference between em and rem lies in their method of referencing. Em units inherit the font size from their parent elements, making them prone to cascading effects. Rem units, on the other hand, maintain a consistent relationship with the base font size, regardless of nesting. This attribute makes rem particularly useful for maintaining consistent font sizes across different levels of the webpage hierarchy.

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